The electrokinetic properties of colloidal magnetic iron oxides.

A novel electrokinetic streaming potential technique has been used to determine the ζ potential behavior of three magnetic iron oxides, (Fe(3)O(4), γ-Fe(2)O(3), and CoFe(2)O(4)) as a function of pH and salt concentration. These colloidal materials, (nanosize in one dimension), are held in the form of a plug by means of external magnets. The streaming potential (E) is measured as a function of fluid flow induced by a pressure drop (ΔP) across the plug. The magnetically held plug is found to obey the requirements of the streaming potential technique; in each case an iso-electric point, (iep) independent of salt concentration is observed. However, if one uses the appropriate quantities in the standard formula, the calculated ζ potentials are very much lower than for oxides such as silica, alumina or goethite and other colloidal oxide, latex, etc. particulates in aqueous salt solutions. Furthermore, at a given pH, the measured ζ potentials anomalously increase in magnitude rather than decrease as observed conventionally as the salt concentration is increased. This apparent anomalous behavior could not be eliminated by incorporating surface conductance effects. However by including a conductance pathway, independent of pH or salt concentration, through the magnetic particle network itself, the anomaly was removed. Confirmation of the role of a conductance pathway through the magnetic particle network was obtained by using silica coated magnetic particles which displayed normal electrokinetic behavior. Finally, we have redesigned the plug-electrode assembly to allow measurement of streaming current, a technique know to eliminate contributions from plug network conductances of any kind. The resulting ζ potentials, derived from this streaming current technique are normal.